1. Technical Field
The present invention generally relates to an image forming apparatus, such as, a copier, a printer, a facsimile machine, a plotter, or a multifunction peripheral (MFP) including at least two of coping, printing, facsimile transmission, plotting, and scanning capabilities and, more particularly, to an image forming apparatus to transfer a toner image formed on an image bearer onto a recording medium.
2. Description of the Background Art
In electrophotographic image forming apparatuses, generally image density fluctuates depending on environmental changes (such as changes in temperature and humidity) or changes (e.g., degradation) over time. Therefore, many electrophotographic image forming apparatuses execute image density adjustment at a predetermined timing to maintain a constant image density. In typical image density adjustments, a gradation pattern, constructed of multiple toner patches that differ in target image density, is formed on an image bearer such as a photoreceptor, and the density of each toner patch is detected by an image density sensor such as an optical sensor. Then, based on detection results (outputs from the density sensor) of each toner patch, image forming conditions such as exposure energy (exposure power), charge bias, and development bias are changed so that a target amount of adhering toner can be attained with a specific image density. Additionally, the concentration of toner in developer used as a control referent is changed as required to adjust the concentration of toner in developer.
Optical sensors including a light-emitting element, such as a light-emitting diode (LED), and a light-emitting element, such as a phototransistor, are often used as the density sensor for detecting the amount of toner adhering to (i.e., amount of adhering toner) each toner patch forming the gradation pattern. Generally, as such optical sensors, there are three types of sensors, those to detect specular reflection light only, those to detect diffuse reflection light only, and those to detect both types of light. To detect the amount of toner adhering to each toner patch forming the gradation pattern using the optical sensor, the gradation pattern is formed on a surface (a surface to be detected) of a bearer (hereinafter “pattern bearer”), such as an image bearer and sheet conveyance member, configured to bear the gradation pattern, and the LED light is directed to the each toner patch carried on the pattern bearer. Then, the light-receiving element detects light reflected (specular reflection or diffuse reflection) therefrom, and the result of detection (outputs from the optical sensor) is converted into the amount of toner adhering to each toner patch.
To detect the amount of toner adhering to each toner patch accurately using such an optical sensor, it is preferred that the light-receiving element of the optical sensor receive only the light reflected from the toner patch. In other words, it is preferred that the light received by the light-receiving element of the optical sensor does not include light reflected from the background on the surface to be detected, where the toner patch is not present. For that, the toner patch should be greater than a spot diameter of light, applied by the light-emitting element, on the surface to be detected.
However, a positional deviation may be caused between the position of the toner patch on the surface to be detected and the position where the optical sensor is disposed due to tolerances in manufacturing or assembling. Accordingly, the length of the toner patch in the direction in which the surface of the pattern bearer moves (hereinafter simply “length of the toner patch”) is made longer than the spot diameter so that the spot diameter falls within the toner patch at the time of the measurement by the optical sensor, even if such a deviation is present.
By contrast, as the length of the toner patch increases, the amount of toner used to form the toner patch increases, resulting in increases in frequency of replacement of a waste-toner container and the running cost of the image forming apparatus. Further, as the amount of toner removed in removal of the toner patch increases, the load on a cleaning member increases, and the operational life of the cleaning member is shortened. Therefore, the length of the toner patch is preferably shorter regarding this inconvenience.
In an image forming apparatus proposed in JP-2007-316237-A, before forming a density patch (toner patch), a proper position at which a density patch is to be formed is calculated so that a detection range of a density sensor falls within the density patch. In this image forming apparatus, initially, a toner pattern for position detection (i.e., a position-detecting pattern) is formed on an image bearer and detected by the density sensor. Then, based on the detection results, the proper position for the density patch (an offset amount from a reference position of the density patch) is calculated. After the proper position of the density position is calculated, the density patch is formed at the calculated position and detected by the density sensor, and image density adjustment is performed based on the detection results.
According to JP-2007-316237-A, the density patch can be formed at a position adjusted in view of the above-described deviation, and it is not necessary to increase the length of the density patch in view of the deviation. Thus, the density patch can be shorter.